Delignification and bleaching of cellulose pulp with oxygen and alkali in the presence of magnesium chloride inhibitor

ABSTRACT

Delignification and bleaching of cellulose pulp with oxygen and alkali by blending the cellulose pulp with an inhibiting quantity of magnesium chloride and then with alkali prior to heating with oxygen. The mixture of magnesium chloride and alkali precipitate finely divided and well dispersed magnesium hydroxide in situ within the cellulose pulp stabilizing the cellulose during the delignification process.

United States Patent Aung et al.

[ Feb. 13, 1973 DELIGNIFICATION AND BLEACHIN G OF CELLULOSE PULP WITH OXYGEN AND ALKALI IN THE PRESENCE OF MAGNESIUM CHLORIDE INHIBITOR FOREIGN PATENTS OR APPLICATIONS 683,771 7/1969 South Africa ..l62/65 OTI-IER PUBLICATIONS Millor, Inorganic And Theoretical Chemistry, Vol. 4, p. 290.

Primary Examiner-Robert L. Lindsay, Jr.

Assistant ExaminerArthur L. Corbin Attorney-Griswold & Burdick, D. B. Kellom and Richard W. Hummer [57] ABSTRACT Delignification and bleaching of cellulose pulp with oxygen and alkali by blending the cellulose pulp with an inhibiting quantity of magnesium chloride and then with alkali prior to heating with oxygen. The mixture of magnesium chloride and alkali precipitate finely divided and well dispersed magnesium hydroxide in situ within the cellulose pulp stabilizing the cellulose during the delignification process.

3 Claims, No Drawings DELIGNIFICATION AND BLEACHING OF CELLULOSE PULP INTO OXYGEN AND ALKALI IN THE PRESENCE OF MAGNESIUM CHLORIDE INHIBITOR BACKGROUND Delignification and bleaching of cellulosic pulps with air or oxygen in an alkaline medium can be used in place of chlorination and alkaline extraction in the first stage purification of raw cellulose pulps. Harris US. Pat. No. 2,673,148 utilizes weak acid salts, such as sodium bicarbonate, sodium tetraborate, sodium benzoate, etc., as a buffering agent in the alkaline pulping of cellulosic raw materials with oxygen at a controlled pH of about 7 -9. Robert, Traynard and Martin- Borret US. Pat. No. 3,384,533 describes the delignification and bleaching of cellulose pulps with oxygen in an alkaline medium in the presence of about 0.5-3.0 weight percent of an alkaline earth carbonate or certain other inorganic compounds, all having a water solubility of less than about 0.1 weight percent at 25C. These additives, preferably magnesium carbonate but also including barium carbonate, calcium carbonate, zinc carbonate, alkali metal botates, titanium dioxide, reduce hydrolysis and degradation of the cellulose polymer during delignification thus improving the physical and mechanical properties of the treated pulp.

STATEMENT OF THE INVENTION In further studies of the alkaline oxygen delignification and bleaching process, it has been discovered that improved results are achieved when the cellulose pulp is blended with an aqueous solution of a magnesium salt having a water solubility of at least 1.0 weight percent at 25C. By using an aqueous solution of a more water soluble magnesium inhibitor with and within the pulp is achieved. Reaction of the magnesium salt with alkali precipitates in situ within the cellulose an insoluble magnesium hydroxide which reduces the degradation of the cellulose during the oxygen-alkali delignification and bleaching process. Preferably the cellulose pulp is blended with about 0.05-1.0 weight percent of the magnesium inhibitor calculated as Mg based on dry weight of the cellulose pulp prior to the addition of alkali.

GENERAL DESCRIPTION In the process of Robert, Traynard and Martin-Borret US. Pat. No. 3,384,533, a limited group of alkaline earth carbonates all having a water solubility of less than about 0.1 weight percent at 25C are used as catalytic inhibitors or stabilizers during the alkaline oxygen delignification and bleaching of cellulosic pulps. Magnesium carbonate is the preferred inhibitor. Typically, the reactor is charged with cellulose pulp, aqueous caustic, oxygen and the inhibitor. Then it is heated at 90-l30C for 0.5-3 hours and the treated pulp discharged for further processing. Tests of the pulp treated in the presence of the inhibitor show improved physical and mechanical characteristics because of reduced cellulose degradation.

Further study of this alkaline oxygen delignification and bleaching process revealed that improved results are achieved by treating the cellulose pulp with an aqueous solution of a magnesium salt having a water solubility greater than 1.0 weight percent at 25C. Use of an aqueous solution containing l-lO percent or more of the soluble magnesium inhibitor salt gives a more uniform distribution and adsorption of the magnesium inhibitor within the treated cellulose pulp. Particularly effective is the pretreatment of the cellulose pulp with the aqueous magnesium salt solution prior to addition of the alkali. Then when the alkali is added, a

well dispersed, insoluble magnesium hydroxide is precipitated in situ within-the cellulose pulp as inhibitor. With very effective mixing of the cellulose pulp, the alkali can be added before or concurrently with the soluble magnesium salt.

Suitable magnesium salts for this improved process include magnesium sulfate, magnesium chloride, magnesium nitrate, and magnesium acetate in anhydrous or hydrated forms. Because of its compatibility with Kraft pulping systems, magnesium sulfate is a preferred soluble magnesium compound.

The high water solubility of these salts permits preparation of solutions containing l-l0 percent or more of the magnesium salt for blending with the cellulose pulp. Sufficient magnesium solution must be used to give the treated pulp magnesium concentration of at least about 0.05 weight percent Mg, calculated'as Mg and based on dry weight of pulp. Normally a Mg concentration of about 0.2-0.6 weight percent is preferred, although higher concentrations can be used within economic limits.

In the preferred practice, the cellulose pulp is pretreated with an aqueous solution of the soluble mag-- nesium salt by vigorous blending. Then the sodium hydroxide is added precipitating a finely divided, well dispersed magnesium hydroxide within the cellulose pulp. Normally a caustic concentration of about 2-15 weight percent based on dry pulp is used. After removing excess water from the pulp mixture, it is heated with about 5-10 atmospheres of oxygen at l30C for 0.53.0 hours to achieve delignification and preliminary bleaching of the raw pulp. Then the oxygen pressure is released, and the pulp is cooled and washed before analysis and further processing.

The following examples illustrate further the improved process. Unless otherwise indicated, all parts and percentages are by weight.

EXAMPLE 1 Alkaline Earth Inhibitor Test Samples of a 77% soft wood/23% hard wood unbleached kraft pulp having a pulp consistency of 20 percent were blended thoroughly with aqueous solutions of a series of soluble alkaline earth salts. For comparison, other samples were blended with aqueous slurries of magnesium carbonate and magnesium basic carbonate (hydromagnesite). Sufficient aqueous caustic was added to each pulp sample to give 7 percent residual NaOH based on dry pulp after removing excess water to obtain a 20 percent pulp consistency. After thorough mixing at room temperature, the excess aqueous liquid decanted, the pulp consistency adjusted to about 20 percent, and the resulting cellulose pulp fluffed and loaded into a reactor. The reactor was evacuated, pressured with oxygen, and heated for 1 hour at C with an oxygen pressure of about psig (8.2 atmospheres). The reactor was vented and cooled. Then the pulp sample was recovered, thoroughly washed with water, and air dried. I

The test samples were evaluated using standard test methods to determine the brightness (GE Brightness 5 using calibrated reflectance meter), pulp viscosity as a 0.5 percent solution in cupriethylenediamine (TAPPI Method T 230 su-66), and residual pulp unsaturation (Kappa No., TAPPI Method T 236 m-60). The effectiveness of the inhibitors is shown in these tests by a higher final pulp viscosity coupled with reduced residual unsaturation (lower Kappa No.).

A. Typical results for a number of alkaline earth salts are given in Table l. The cellulose degradation in the absence of an inhibitor is shown in Test l-l while Test l2 is typical of results with an insoluble magnesium carbonate. The superiority of the magnesium salts, and particularly of the water soluble magnesium salts is evident. A further advantage of the soluble salts, the

relative ease of blending the pulp with the aqueous test solutions, was noted in preparing the test samples.

TABLE 1 Alkaline Earth Inhibitor Test (7% NaOH, 120 psi, 1 hr, 95C) Concenlrauon" Test Inhibitor G. E. Visc., Kappa Salt mg Brightcps No [less l-0 none (ini r1111 pulp) 28.0 18.6 33.8 l-l None 0 0 51.4 4.1 6.3

(Control) 1-2 3 M co *Mg(OH)- 314 0" 1.0 0.27 52.8 7.5 6.4 1-3 BaCl -21-1.,o 1.5 52.0 3.1 6.7 I-4 SrCl (EH-,0 1.5 51.5 3.6 6.8 1-5 MgCl 611.0 1.5 0.29 49.8 7.9 6.8 1-7 M so 1.5 0.30 49.9 8.1 6.0

a. Inhibitor concentration based on dry cellulose pulp b. Magnesium basic carbonate (hydromagnesite) B. Using the same pulp, test procedure and 4 percent sodium hydroxide based on dry cellulose pulp, results in Table 2 show the effectiveness of the soluble magnesium sulfate and acetate inhibitors.

TABLE 2 6O Magnesium Inhibitors (4% NaOH, 120 psi, 1 hr., 95C) Concen- G.E. tration" Bright- Visc., Kappa Test Inhibitor %Mg ness p N v salt g 2-0 none(ini- 28.0 18.6 33.8

tial pulp) 2-1 none (con- 0 0 49.0 7.4 6.9

trol) 2-2 MgCO -3H O 1.5 0.29 47.8 10.3 7.2 2-3 MgSO 1.5 0.30 45.0 10.0 8.4 2-4 MgSQ," 1.5 0.30 45.8 10.3 7.9 2-5 MgSO '7H,O 3.1 0.30 46.4 9.9 7.6 2-6 Mg(OAc) a. Inhibitor concentration based on dry cellulose pulp b. Calcined Kieserite (MgSO -H O) EXAMPLE 2 Magnesium Concentration Using the same pulp and general procedure of Example 1, the effect of the magnesium salt concentration was studied with 4 percent sodium hydroxide based on cellulose pulp. Typical results are given in Table 3.

TABLE 3 Mg Concentration (4% NaOI-I, 120 psi, 1 hr, C)

Concentration G. E. Test MgSQ, Mg Bright- Visc., Kappa ness cps No. 3-0 28.0 18.6 33.8 3-1 0 0 49.0 7.4 6.9 3-2 0.5 0.1 46.5 9.2 7.7 33 1.0 0.2 44.4 9.8 8.1 3-4 1.5 0.3 45.0 10.0 8.3 35 2.0 0.4 45.4 10.4 8.3 3-6 3.0 0.6 46.2 10.0 7.5 3-7 5.0 1.0 44.6 10.0 7.9

Example 3 Magnesium Inhibitor with Hardwood Pulp Table 4 presents data on tests using the general procedure of Example 1 and a percent hardwood unbleached kraft pulp and 4 percent sodium hydroxide based on cellulose pulp.

TABLE 4 Soluble Mg Salts with Hardwood Pulp (4% NaOH,

psi, 1 hr, 95C) Visc Test Inhibitor %Mg G. E. cps

kappa Salt Mg Bright No.

ness 4-0 None (Initial pulp) 30.6 24

14.0 41 None (Control 0 O 58.2 10.5 7.2 4 2 3MgCO 'Mg(OH) 1.0 0.27 57.8 13.1 6.9

31-1 0 4-3 MgSO 1.5 0.30 54.4 14.5 7.4 4-4 MgC1 -6H O 2.5 0.30 53.5 13.5 7.1 4-5 Mg(NO;,) '6H O 3.1 0.29 53.5 14.1 7.2

EXAMPLE4 Magnesium Inhibitor with Softwood Pulp Table 5 presents data on tests using the general procedure of Example 1 and a 100 percent softwood (high yield) kraft pulp and various percentages of sodium hydroxide based on cellulose pulp.

TABLE 5 Mg inhibitor with high yield krul't pulp (120 p.s.i., 1 hr., 115 C.)

B. Contacting the blended mixture of cellulose pulp,

magnesium inhibitor and alkali with about 5-1 at- Percent Percent G. E. Vise, Kappa Test NaOH Inhibitor Mg salt Mg brightness c.p.s. number None (Initial) 16.4 22 14 None 0 0 45.4 4.8 16.1 10 MgSO4 1.5 0.30 41.4 0.0 19.4 8 None 0 0 40.6 6.4 22.0 8 MgSOi 1.5 0.30 35.7 10.8 26.7 4 None. 0 0 29.5 12.8 36.4 4 MgSO4 1.5 0.30 28.1 15.4 30.2

Tests with other pulps confirmed the utility of the soluble magnesium salt as inhibitors or stabilizers during the alkaline oxygen delignification process and the further advantage of the improved process in the ease of treating the pulp to obtain a more uniform dispersion of the magnesium inhibitor throughout the treated cellulose pulp.

We claim:

1. in a process for delignification and bleaching of cellulose pulp by treatment with oxygen in an alkaline medium, the improvement which comprises:

A. Blending the cellulose pulp with an inhibiting quantity of magnesium chloride, and then with about 2-15 weight percent alkali based on dry cellulose pulp; and 

1. In a process for delignification and bleaching of cellulose pulp by treatment with oxygen in an alkaline medium, the improvement which comprises: A. Blending the cellulose pulp with an inhibiting quantity of magnesium chloride, and then with about 2-15 weight percent alkali based on dry cellulose pulp; and B. Contacting the blended mixture of cellulose pulp, magnesium inhibitor and alkali with about 5-10 atmospheres of oxygen at about90*-130*C to delignify and bleach the cellulose pulp.
 2. The process of claim 1 where the cellulose pulp is blended prior to the addition of the alkali with an aqueous solution of the magnesium inhibitor to give an inhibitor concentration in the blended cellulose of about 0.05-1.0 weight percent Mg based on dry cellulose pulp. 